1. Field of the Invention
The present invention relates to a variable capacitance element included in a high-frequency circuit. In particular, the present invention relates to a variable capacitance element, for use as, for example, a variable capacitance switch or a variable capacitor. In this case, the variable capacitance switch performs switching operations on high-frequency signals by changing electrostatic capacitance.
2. Description of the Related Art
Generally, a variable capacitor is known. A variable capacitor displaces, for example, a movable electrode with respect to a fixed electrode by using electrostatic gravity so as to change the spacing between these electrodes. Thus, the electrostatic capacitance is variably selected.
This kind of conventional variable capacitor is substantially similar to an electrostatically-driven switch as disclosed in Japanese Unexamined Patent Application Publication No. 2000-188050. The variable capacitor includes a movable electrode on the right side of the substrate including a flexible supporting bar. The flexible supporting bar bends toward the front side of the substrate. The movable electrode is spaced from and faces toward the fixed electrode provided on the substrate. Driving electrodes are provided on the substrate side and the movable electrode side. Voltage is applied between the driving electrodes externally such that electrostatic gravity is produced.
When voltage is not applied between the driving electrodes, the supporting bar freely supports the movable electrode. Thus, a predetermined space (electrostatic capacitance) is set between the fixed electrode and the movable electrode. When voltage is applied between the driving electrodes, the supporting bar is bent and is deformed due to the electrostatic gravity such that the movable electrode is displaced toward the fixed electrode. Thus, the electrostatic capacitance between these electrodes increases.
FIG. 4A schematically shows an example of a shunt switch element, which is a variable capacitance element. A shunt switch element 130 includes a substrate 131 containing a dielectric. A coplanar line 132 is provided on the substrate 131. High-frequency signals are conducted through the coplanar line 132. Three lines 133g1, 133s and 133g2 are aligned at a desired interval on the substrate 131. The middle line 133s is a signal line. The lines 133g1 and 133g2 on both sides of the signal line 133s are ground lines.
On the coplanar line 132, both ends of an electrode bridge 134 are joined with the ground lines 133g1 and 133g2. The electrode bridge 134 crosses over the signal line 133s. FIG. 4B is a top view of FIG. 4A schematically showing the coplanar line 132 and the electrode bridge 134.
When direct-current voltage is applied between the signal line 133s and the electrode bridge 134 included in the shunt switch element 130, electrostatic gravity is produced between the signal line 133s and the electrode bridge 134. As a result, the electrode bridge 134 is pulled toward the signal line 133s due to the electrostatic gravity. Therefore, the electrostatic capacitance changes between the electrode bridge 134 and the signal line 133s of the coplanar line 132.
It is important to note that equivalent circuits of the coplanar line 132 and the electrode bridge 134 can be expressed as shown in FIG. 4C. In FIG. 4C, the reference letter C indicates an electrostatic capacitance between the signal line 133s and the electrode bridge 134. The reference letter L indicates an inductance component of the electrode bridge 134. The reference letter R indicates a resistance component of the electrode bridge 134.
When the space between the signal line 133s and the electrode bridge 134 is reduced, and when the electrostatic capacitance C between the signal line 133s and the electrode bridge 134 is increased, the self-oscillating frequency of the LC series circuit in FIG. 4C decreases. At the self-oscillating frequency of the LC series circuit, the impedance of the LC series circuits is minimized. Thus, when viewing the ground lines 133g1 and 133g2 from the signal line 133s through the electrode bridge 134, a short circuit occurs at high frequencies of the self-oscillating frequencies of the LC series circuit. As a result, the conducting of high frequency signals through the coplanar line 132 (signal line 133s) is turned OFF.
On the other hand, when the space between the signal line 133s and the electrode bridge 134 is increased and when the electrostatic capacitance C between the signal line 133s and the electrode bridge 134 is decreased, the self-oscillating frequency of the LC series circuit in FIG. 4C increases. As a result, when viewing from the signal line 133s through the electrode bridge 134, the ground lines 133g1 and 133g2 are open to high frequencies. Therefore, the conducting of high-frequency signals through the coplanar line 132 is turned ON.
As described above, the shunt switch element 130 controls ON and OFF of the conducting of high frequency signals through the coplanar line 132. In this case, the electrode bridge 134 is displaced and the electrostatic capacitance C is variable between the electrode bridge 134 and the signal line 133s. 
It is important to note that, in the above-described conventional technology, the movable electrode is held at a predetermined location by its own force (spring force) when voltage is not applied between the driving electrodes. However, when a variable capacitance element is used, an external force such as vibration and impact may be applied to the substrate. Thus, when external forces act thereon in a direction perpendicular to the substrate, the supporting bar is bent and is deformed by the external force. As a result, the movable electrode is displaced with respect to the fixed electrode.
Therefore, while the variable capacitor is operating, the electrostatic capacitance of the capacitor may be changed due to vibration and/or impact even without voltage being applied to the driving electrode. Therefore, the vibration resistance and reliability are reduced. This is a disadvantage.
Furthermore, in the conventional variable capacitance element, when two driving electrodes are too close to each other and contact each other, they remain fixed together even after the voltage is terminated. In this case, returning the variable capacitance element to the normal operating state is difficult using only the stability of the supporting bar. This is another disadvantage.
In the construction of the shunt switch element 130, the electrode bridge 134 functions as both a driving electrode and an electrode for electrostatic capacitance. In this case, the driving electrode is paired with a fixed driving electrode to produce electrostatic gravity. The electrode for electrostatic capacitance is paired with the signal line 133s to determine the self-oscillating frequency of the LC series circuit shown in FIG. 4C.
However, when high frequency signals are conducted through the coplanar line 132, such as extremely high frequency (EHF) signals, the amount of electrode surface area of the electrode bridge 134 must be reduced. Thus, the conducting of high frequency signals through the coplanar line 132 can be turned ON and OFF precisely by using changes in the self-oscillating frequency of the LC series circuit using the change in electrostatic capacitance as described above. On the other hand, when the electrode bridge 134 is small, large direct-current voltages must be applied between the electrode bridge 134 and the fixed driving electrode in order to produce electrostatic gravity for displacing the electrode bridge 134. However, the electrode bridge 134 is preferably displaced by using low direct-current voltage. Therefore, from the viewpoint of the displacement driving, the size of the electrode bridge 134 is preferably increased.
As described above, the size of the electrode bridge 134 for controlling ON and OFF of the conducting of high frequency signals is different from the size of the electrode bridge 134 that is suitable for displacement of the electrode bridge 134 itself. Thus, designing the electrode bridge 134 is difficult.